The project I need help on is a low power, battery operated system that incorporates a sensor array taking periodic measurements. The premise is quite simple, but I'm lacking the practical experience to complete the design. I have specific questions, but overall feedback is welcome as well.
Requirements
Design
The attached schematic is the interface circuit for each sensor in the array. Note that this circuit is designed to daisy chain the sensors with each connecting to the next.
The sensor array is normally powered off to conserve power and armed via a (closed) reed switch found on the schematic. Opening the reed switch causes the following:
The sensors are interfaced to the MCU via a UART operating in SPI mode with the MOSI pin connected to LOAD and MISO connected to DOUT of the first sensor. Transmitting a special pulse byte loads a consistent count into the shift register after which the counts can be shifted out down the line.
Oh, I'm only using the low 6 bits of the count and hijacking the high order bits for a little self diagnostic capability.
Questions/Concerns
I've done quite a bit of reading and searching, but to no avail. Please help
Requirements
- Detect an initiating event using a normally closed circuit
- Count discrete events from an array of open collector sensors and periodically report this information back to an MCU (8051 derivative). The reported count must be consistent i.e. sampled at the same point in time across all sensors.
- Consume the lowest possible power for extended battery operation (2-3 AAs)
- Sensor bus length of up to 3 meters with sensors stationed along the length of the bus with up to 1 meter distance between sensors and/or MCU
- Accommodate between 4 and 16 sensors on the bus
- Operate in an office/industrial environment (semi-noisy)
- Minimal sensor array thickness. Clearances are tight.
Design
The attached schematic is the interface circuit for each sensor in the array. Note that this circuit is designed to daisy chain the sensors with each connecting to the next.
The sensor array is normally powered off to conserve power and armed via a (closed) reed switch found on the schematic. Opening the reed switch causes the following:
- Interrupt on the MCU
- Power up the sensor array to start counting events
- Periodic reading taken
The sensors are interfaced to the MCU via a UART operating in SPI mode with the MOSI pin connected to LOAD and MISO connected to DOUT of the first sensor. Transmitting a special pulse byte loads a consistent count into the shift register after which the counts can be shifted out down the line.
Oh, I'm only using the low 6 bits of the count and hijacking the high order bits for a little self diagnostic capability.
Questions/Concerns
- Noise and overall reliability of the circuit. I mentioned the operating environment, but the circuit must also withstand plugging/unplugging the array without damage. Of course, I'm not comfortable directly connecting to the MCU I/O pins, but I don't know specifically how to address.
- Providing power to the array. Each sensor in the array will consume up to 1 mA, which isn't a lot, but once again, I'm not comfortable with a direct connection to the MCU. I was thinking of using a standard bipolar transistor to switch and have read about JFETs and MOSFETs. This should be a simple, standard problem - any suggestions?
- Clock distribution. Should I be using a buffer or something on this? Also, I'm assuming I'll have to fine tune the RC values (R4) based on the actual cable used, but is there a better approach? I'd like to run the bus at 100kHz max.
- Normally closed interrupt (INT). The idea here is to consume very little current when not operational. The current design (not shown) connects the interrupt line to the MCU and uses a 250k pullup. I'm concerned about a direct ground loop connection to the MCU as well as parasitic losses through the pullup (perhaps an unwarranted concern).
- Cabling. I'll likely need two types. Clearances seem to require flat cable between sensors while routing to the MCU requires flexibility. Any suggestions/experience with cabling and connectors?
- Surface mount prototyping. I'm currently using a breadboard to prototype, but is there a good SMT prototyping system. I've seen a couple, but none seem all that great.
- Schematics. What's a good free/low-cost schematic package for simple stuff like this? I'm currently using ExpressSCH since I'll probably be using them to do prototype boards at some point.
I've done quite a bit of reading and searching, but to no avail. Please help
Attachments
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